The present invention relates to the field of vacuum cleaning and in particular the collection of air-entrained debris, for example in a filter bag.
Domestic and upright type vacuum cleaners are typically equipped with porous paper bags into which dirt-laden air lifted from a floor or carpet is drawn by a vacuum drive. Cyclonic separators are also well known in both a domestic and industrial contexts for separation of air-entrained particles from an air stream.
Industrial and commercial bin-type cleaners frequently avoid using a bag for collection, so as to avoid bag replacement. In this case dirt is drawn from a floor surface into a tubular wand and collected directly in a drum or bin rather than a bag. A filter cup or layer may be used to cover a vacuum drive port typically in an upper end region of the bin. The bag prevents fine particles from leaving the bin during use and fouling the vacuum drive (typically an electric motor and impeller combination.) For wet collection the filter bag may be made from a plastics material or foam, so that filtering performance is not compromised by airborne liquid.
A particular problem exists in the field of vacuum cleaners used in building or construction sites to collect fine brick or plaster dust which is created when these materials are subject to grinding by tools. For example, when channels or tracks are formed in walls or partitions of buildings so as to permit the chasing-in of pipe work or electric cabling or the like into the wall. Large volumes of very fine dust are generated by such processes. So as to avoid contaminating the building interior with dust, vacuum cleaner attachments are provided for the grinding tools so that the dust may be collected. In addition health and safety requirements mean that dust contamination should be minimized. Thus external air recirculation filters may be used in combination with the vacuum cleaners.
One problem which arises in the collection of fine brick or plaster dust is that the upstream internal surfaces of collection bags or filters rapidly become blocked with a surface layer of dust. Even high performance multi-layer collection bags rapidly become saturated with dust which prevents airflow through the bag or filter and therefore markedly reduces the vacuum pressure drop. It has been found that a period of five to ten minutes of suction is all that can be maintained when collecting brick or plaster dust during chasing-in grinding or cutting.
To overcome this, two approaches have been adopted. One is to do without collection bags and use high surface area star-plan filters in the vacuum path next to the drive motor. These filters may be agitated or vibrated so as to displace the caked-on dust. The agitators are not very effective because detached dust particles remain airborne and a thus rapidly drawn back onto the filter surface. A serious problem in bagless cleaners is the emptying of the collection bin. As there is no bag there is a danger that dust will be released into the air when the bin is emptied into a refuse sack or the like.
The second approach is for the operator of the bag vacuum cleaner to deactivate the vacuum cleaner when the suction falls too low. This allows the collection bag to contract and “relax” and allow the dust layer formed on the bag interior surface to flake off. Provided the vacuum is turned of for about a minute, allowing the bag to contract and dust to flake off and fall into the lower end regions of the collection bag, then performance will revert to a reasonable level. Nevertheless performance will once again rapidly decrease when the machine is put back into use.
U.S. Pat. No. 6,183,536 discloses a multilayer vacuum cleaner bag made up of a high permeability relatively coarse layer disposed upstream of a fine grade filter. The coarse layer is intended to capture large dust particles and prevent them from shock loading the fine filter layer. The embodiments disclosed are notable for having a low overall thickness of the multi-layer filter, typically less than 1 mm. Although it is claimed that these filters provide extended life in normal working conditions they are unsuitable for use in construction industry applications in which very large quantities of fine brick or plaster dust are to be collected. They would become rapidly caked in dust several millimeters thick as they are unable to cope with the mass of dust produced.
US 2004/0211160 discloses a multilayer filter structure for removing dust from gases. The filter has a fine layer and a coarse layer, the filters each being made up of fibres which have a diameter gradient, so that filter diameter gradually decreases from intake to outlet sides of the filter.
Gebrauchsmuster DE 20 2008 007 717U1 discloses a filter bag in which an inner compartment serves as a pre-filter which is filled first before the rest of the bag.